Up to six longitudinal basement faults are prersent in the zone of border faults of this rift. Their activity has been important in the formation of oil and gas pools.
Fault I on the north and fault II on the south separate the border area of the basement outside the rift from the subsided part within the rift. Fault III separates this subsided part from a structurally higher area within the rift.
The subsided part of the fault zones is more mobile, and within it are chains of downwarps. Faults IV, V, and VI form here smaller horsts and graben. Folds within the fault zones are caused by block faulting and by salt tectonics.
Several groups of highs have been recognized. The first group includes folds associated with the main faults. The second is a chain of brachy-anticlinal highs located between faults I and II or above the latter. The third group includes inter-fault salt-assist highs, in some of which the salt has pierced into the Mesozoic-Cenozoic sediments. The fourth group are folds associated with highs on the Precambrian basement bounded by faults III and transverse faults.
The history of development of the Kholm salt-assist high has been traced, and a relationship established between the faulting and salt tectonics. At the beginning of Zadon- Yeletsa time in the Devonian the single Kholm graben formed, bounded by faults II and VI, and was filled by up to 1850 m of salt. This block movement caused horizontal stresses sufficient for warping the strata of the downdropped block and to initiate salt tectonics.
Beginning with Yeletsa-Lebedyan time the salt began to move up along fault V. Gravity became the drive. At the end of the Devonian the flexure at the base of the Zadon beds was1500 m, and at the beginning of the Paleogene it was 2000 m. Amplitude of the Kholm high increased accordingly to 760 m by Paleogene time.
Salt tectonics controlled by movement of basement blocks along faults II, III, and V played an active role in development of Monastyrishchen high in the zone of the Pripyat-Manych fault. It appears that at the beginning of Zadon-Yeletsa time a graben formed here with amplitude of 1100 m, bounded by faults III and II. It became filled with salt. Then at the beginning of Yeletsa-Lebedyan time movement on fault V was 700 m. Flow of salt during the Late Devonian resulted in a flexure of 1220 m by Tournaisian-early Visean time. Subsequent growth of the fold from Carboniferous to pre- Paleogene time was not significant.
Longitudinal faulting during the Devonian thus led to salt tectonics and development of folds that can act as traps for hydrocarbons.
Taken from Ryabchun, 1986; digested in Petroleum Geology,
Vol. 23, No. 7/8, fourteen cross sections. Copyright 2003
James Clarke. You are encouraged to print out this News
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